Various caliper tools for gauging the diameter of a borehole are known in the art. In one example, a caliper tool includes one or more bow springs coupled to a tool body. When the tool body is disposed in a borehole, the bow spring engages the borehole wall. The bow spring flexes as the tool body traverses the borehole and the borehole diameter changes. The motion of the bow spring can provide an indication of the borehole diameter. In this case, a sensing device can be attached to the bow spring and used to monitor the motion of the bow spring. This is taught, for example, in U.S. Pat. No. 2,639,512. Some caliper tools further include one or more rigid arms coupled between the tool body and the bow spring. The rigid arm deflects as the bow spring flexes, and the motion of the rigid arm provides an indication of the borehole diameter.
An electronic sensing device having a movable part is usually used to monitor the motion of the rigid arm. Typical examples of these electronic sensing devices include linear variable differential transformer (LVDT) and potentiometer sensors. An LVDT sensor includes a ferromagnetic core disposed within a series of inductors and produces electrical output proportional to the physical position of the ferromagnetic core within the series of inductors. A potentiometer sensor includes a slider attached to a resistor and produces electrical output proportional to the contact position of the slider on the resistor. The caliper tool uses a mechanical linkage to couple the movable part of the sensing device to the rigid arm so that the electrical output generated by the sensing device is representative of the motion, or deflection, of the rigid arm.
The mechanical linkage is required to satisfy various requirements. For example, the mechanical linkage is required to fit in a small space on the tool body and work in the hydrostatic pressure of the borehole, which frequently exceeds 20,000 psi (138 MPa), and in the presence of drilling mud, which typically contains debris. The mechanical linkage must be mechanically tight to avoid introducing errors in translating the position of the rigid arm to the sensing device. To allow attachment to the mechanical linkage, the movable part of the sensing device would either have to be exposed to borehole pressure and drilling fluid or be located in a compensator filled with oil at borehole pressure.
As evident from conventional configurations, physically linking the sensing device to a rigid arm complicates the design and operation of a caliper tool. A caliper tool that does not require a mechanical linkage to translate motion of an arm to a sensing device is desired.